1. Field of the Invention
The present invention relates to an equipment and a method for bandwidth allocation at an optical line terminal in the Passive Optical Network.
2. Description of the Related Art
The Internet is rapidly spreading and developing. Accordingly, large amount of data such as image data or video data is often communicated, raising an important issue of improving a communication line to a broadband to allow wideband access to the Internet. As a broadband access line, various types of technology have been put into practical use such as ADSL (Asymmetric Digital Subscriber Line) using existing telephone lines, or a cable modem using a coaxial line of CATV (Cable TeleVision) as a network line. However, for realization of high-speed and high-quality communication environment, it is desired that a communication line is further improved to a wideband. In such status, a PON (Passive Optical Network) as a network using optical fiber is getting attention.
FIG. 11 illustrates a PON configuration and downstream frame transmission. The PON includes an OLT (Optical Line Terminal) 101 located in a station (not shown), first to Nth ONUs (Optical Network Units) 1021-102N located in respective end users' homes and an optical splitter 103 that splits or joins signals among the ONUs. Each personal computer (not shown) in the end user's home connects to the network via any of the ONUs 1021-102N, the optical splitter 103 and the OLT 101.
In the above PON, upstream and downstream signals are wavelength-multiplexed into single, bi-directional optical fiber for transmission. For example, the wavelength of an upstream signal is 1.3 μm in many cases, while the wavelength of a downstream signal is 1.5 μm. As illustrated in FIG. 11, a downstream signal is broadcast from the OLT 101 to all ONUs 1021-102N. The broadcast means simultaneous transmission to all nodes in the same segment. In this case, the first to Nth ONUs 1021-102N check destinations of transmitted frames and capture frames only addressed to themselves.
For example, assume that the OLT 101 transmits frames 1111, 1112, 1113 and 1114 having destination A2, A1, AN and A1, respectively in that order. In this case, the optical splitter 103 simply splits and transmits the frames 1111, 1112, 1113 and 1114 to all ONUs 1021-102N in that order. Assume that a destination to the first ONU 1021 is A1, a destination to the second ONU 1022 is A2, and a destination to the Nth ONU 102N is AN. In this case, the second ONU 1022 first captures the frame 1111 having the destination A2. Next, the first ONU 1021 captures the frame 1112 having the destination A1. Then, the Nth ONU 102N captures the frame 1113 having the destination AN. Finally, the first ONU 1021 captures the frame 1114 having the destination A1.
FIG. 12 illustrates upstream frame transmission in a PON identical to that of FIG. 11 in configuration. Upstream frames transmitted from the first to Nth ONUs 1021-102N join at the optical splitter 103. At the optical splitter 103, it is necessary to avoid a temporal overlap of frames and collision of signals. For this necessity, time division multiplexing communication is performed for the frames sent from the first to Nth ONUs 1021-102N not to overlap. In the time division multiplexing communication, the OLT 101 coordinates an output request (REPORT) reported from the first to Nth ONUs 1021-102N. OLT 101 calculates transmission time based on distances between the OLT 101 and each of the first to Nth ONUs 1021-102N. Then, OLT 101 grants signal sending permissions (GATEs) that specify transmission timing to some of the first to Nth ONUs 1021-102N that have reported the output requests.
Each of the output requests (REPORTs) reported by the first to Nth ONUs 1021-102N includes the state of a queue, i.e. the length of the queue in a buffer memory (not shown) for storing signals to be transmitted for information. According to the length of a frame, the OLT 101 can specify the timing to permit transmission of the frame. A signal sending permission (GATE) output by the OLT 101 includes the time to start sending and the time period to continue sending that depend on the priority of a frame subjected to an output request (REPORT).
Some of the first to Nth ONUs 1021-102N that have performed output requests (REPORTs) (hereinafter called “ONUs 102REP”) send upstream signals according to signal sending permissions (GATEs) sent to them. In other words, bandwidth allocation for the upstream signals from each ONU 102REP is accomplished as allocation of a time slot for upstream signal transmission.
FIG. 13 illustrates a conventional exchange of the above described output request (REPORT) and of the signal sending permission (GATE) to it. In this diagram, the signal exchange between the OLT 101 and the single ONU 102REP is illustrated using a time axis (time) as the axis of abscissas. First, the ONU 102REP sends an output request R1 at time t1 on the occurrence of a signal to be transmitted. When the OLT 101 receives the request, it sends back a signal sending permission G1 and the ONU 102REP receives the permission at time t2. The ONU 102REP starts sending data D1 in a predetermined time slot TS1 at time t3. A period between the time t1 and the time t3 is waiting time W1. The time when the sending of the data D1 completes is time t4.
The ONU 102REP sends a second output request R2 at time t5 after the time t4. The OLT 101 sends back a signal sending permission G2 and the ONU 102REP receives the permission at time t6. The ONU 102REP starts sending data D2 in a predetermined time slot TS2 at time t7 after waiting time W2. The time when the sending of the data D2 completes is time t8. The data D1, D2 . . . are transmitted by repeating the above processing.
Each of the output requests R2, R3, . . . following to the first request R1 can be transmitted piggyback at the ends of the data D1, D2, . . . that are previously sent, respectively. For example, if the second output request R2 is ready to be output at the time t4, the request R2 can be transmitted without waiting till the time t5. In this case, the time t5 is identical to the time t4.
Next, communication between the ONUs 1021, 1022, . . . , 102N and the OLT 101 shown in FIG. 11 is described with reference to FIG. 14. A cycle that transmission of upstream signal transmitted from ONU to OLT makes a circuit of all ONUs 1021-102N is referred to as a Service Cycle (SC). Assume that N equals three. FIG. 14 shows contents of an Mth service cycle SCM and the next (M+1)th service cycle SCM+1 in detail. Usually, the length of a service cycle SC does not need to be always constant, but may change dynamically depending on the status of output requests by the respective ONUs 1021-102N.
An Ethernet (R) PON for transmitting Ethernet packets via a PON is standardized according to the IEEE (The Institute of Electrical and Electronics Engineers, Inc.) 802.3ah. The IEEE 802.3ah defines frame formats for an output request (REPORT) message and a signal sending permission (GATE) message. However, since it does not define an upstream bandwidth allocation method or algorithm, these can be defined in equipment design as necessary.
In the Mth service cycle SCM shown in FIG. 14, EF (Expedited Forwarding) data (DATA) of a class for guaranteeing a delay and a bandwidth is transmitted first, as described below. Then, the data D1 of the first ONU 1021 is transmitted to the OLT 101, and the data D2 of the second ONU 1022 is transmitted to the OLT 101 in the next time slot. Finally, the data DN of the Nth ONU 102N is transmitted to the OLT 101. In this way, when the last data DN in the Mth service cycle SCM is transmitted to the OLT 101, traffic allocation ALM in the next service cycle is performed. In the (M+1)th service cycle SCM+1, the data D1 of the first ONU 1021 is similarly transmitted to the OLT 101 after the EF data, and the data D2 of the second ONU 1022 is transmitted to the OLT 101 in the next time slot. Finally, the data DN of the Nth ONU 102N is transmitted to the OLT 101. In the last interval in the (M+1)th service cycle SCM+1, traffic allocation ALM+1 in the next service cycle is performed. Subsequent allocations are performed in the same way as above.
Now, traffic classification is described. The traffic classification is performed for upstream signals to transmit frames corresponding to a plurality of services according to priorities. Each class has a corresponding priority. For example, in the DiffServe (Differentiated Services) of the IETF (The Internet Engineering Task Force), in addition to the EF shown in FIG. 14, classes of AF (Assured Forwarding) and BE (Best Effort) are defined. The EF is a class for guaranteeing a delay and bandwidth and of the highest priority. The AF is a class for not guaranteeing a delay but guaranteeing a bandwidth. The BE is a class for not guaranteeing a delay or a bandwidth and of the lowest priority. Representative service applications of the EF, AF, and BE classes include VoIP (Voice over IP), file transfer and usual Internet access, respectively.
There are several algorithms for bandwidth allocation. There is an algorithm called D1 for the PON (for example, see Y. Luo et al., “Bandwidth Allocation for Multiservice Access on EPONs,” IEEE Communications Magazine 2005 February s16-s21) The D1 algorithm determines in advance a maximum value of a service cycle SC and allocates a bandwidth so as not to exceed the maximum value. First, the EF, a class for guaranteeing both a delay and a bandwidth in an output request (REPORT) by each of the ONU 1021, the ONU 1022 and the ONU 102N is assigned a fixed bandwidth. The remaining bandwidth is allocated to the AF data, a class for not guaranteeing a delay but guaranteeing a bandwidth in an output request (REPORT) by each of the ONU 1021, the ONU 1022 and the ONU 102N. In this allocation, if a total sum of requested AF data is equal to or less than the remaining bandwidth of a service cycle after allocating the EF data, all the requested AF data are assigned.
After the AF data is assigned, if further band width remains in the service cycle SC, the BE data, a class for not guaranteeing a delay or a bandwidth in the output request is allocated. If a total sum of the requested AF data exceeds the remaining bandwidth after the EF data is assigned, the ONUs 102 that requested the AF data transmission are equally assigned the AF data. The BE data is not assigned since the remaining bandwidth is depleted by the ONU 102.
The bandwidth calculation and allocation are performed after the output requests (REPORTs) are notified from all the ONUs 1021-102N to the OLT 101. Based on the bandwidth calculation and allocation, the OLT sends a signal sending permission (GATE) to the relevant ONUs 102.
FIG. 15 illustrates a configuration of bandwidth allocation control unit used in a conventional OLT. A bandwidth allocation control unit 121 includes an allocation module 122 that allocates bandwidth and an interface module 125. The interface module 125 receives output requests (REPORTs) 1231-123N and transmits signal sending permissions (GATEs) 1241-124N from/to the first to Nth ONUs 1021-102N. The interface module 125 receives the state of a buffer memory for storing signals to be transmitted by the first to Nth ONUs 1021-102N, as output requests 1231-123N respectively. When the allocation module 122 has allocated bandwidth, the interface module 125 transmits the result as the signal sending permissions 1241-124N to the ONUs 1021-102N, respectively. The allocation module 122 performs the allocation according to the above described D1 algorithm. The interface module 125 notifies the allocation module 122 of queue state signals 126 indicating the states of queues in buffer memories in the first to Nth ONUs 1021-102N, i.e. queue lengths. The interface module receives allocation complete signals 127 corresponding to the notification and sends the signal sending permissions 1241-124N.
Since the D1 algorithm is relatively simple, D1 is easily implemented in a small-scale PON. Bandwidth is allocated evenly among the ONUs 1021-102N.
However, the conventional algorithm has a problem of system scalability. That is, in this algorithm, after the output requests 1231-123N are collected from the first to Nth ONUs 1021-102N, the allocation is performed intensively till the next service cycle SC starts. Accordingly, if the number of ONUs 102 increases, the allocation module 122 is assigned excessive loads. By this reason, in a large-scale PON, the bandwidth allocation control unit 121 needs an expensive and high-speed integrated circuit or a CPU, which raises the cost of a system. Additionally, if sufficient time is allowed for the bandwidth allocation to solve this problem, starting time of the service cycle SC delays and bandwidth is wasted. As a result, there is a problem that the bandwidth allocation is constrained particularly for the AF data and BE data classes and system performance degrades.
An object of the present invention is to provide an equipment and a method for bandwidth allocation in an optical line terminal that can allocate bandwidth evenly among respective ONUs constituting a PON and does not cause a decrease of bandwidth efficiency for a large number of ONUs.